The invention pertains to semiconductor devices, and, more particularly, to semiconductor devices modified to enhance the capabilities of machine vision systems for automated assembly.
At the heart of an integrated circuit is a semiconductor die. To facilitate handling, the die is sometimes fastened to a supporting carrier. In addition to providing stability, the carrier has larger terminations that can be more easily soldered to other circuit components, e.g., on a printed circuit board (PCB). Another packaging approach which is commonly referred to as xe2x80x9cflip chipxe2x80x9d packaging generally contemplates forming solder bumps (or other suitable contacts) directly on the integrated circuit die, and soldering the flip chip directly to the printed circuit board. When a flip chip or semiconductor package is attached to the substrate, an air gap remains between flip chip and substrate. This gap is commonly filled with a material that is typically a mixture of a resin and small silica spheres and is generally referred to as underfill. The underfill material aids in bonding the chip or package to the PCB.
Machine vision systems play an important role in automated assembly systems. Cameras are used to obtain images of articles, and image processing is performed to identify features of the article. Further image processing may be performed to identify the article""s position, measure its dimensions, and/or to check for article defects. Image processing results may then be used to aid in the control of automated systems, such as factory equipment, including, for example, an industrial controller, a robotic arm, or a positioning table. Present methods of high speed electronic assembly require that the underfill be applied to the flip chip or the semiconductor package prior to attachment to the PCB. The coating thickness of the underfill is one of the most important factors that affects assembly yield and reliability. Underfill coating thickness should be comparable to the solder bump height in order to have sufficient underfill fillet formation after solder reflow to the PCB. However, coating to the solder bump height pushes the limits of the vision system on the placement machines. The performance of the machine vision system is highly dependent on the exposed area of the solder bump and, more accurately, the contrast between the bump and the underfill. With underfill coated chips the bump exposed area is reduced and the bump is difficult to optically measure, which makes the inspection of coated parts prior to placement difficult. Sometimes the vision system can""t recognize the solder bump at all if the coating covers the bump or if the bump open area is beyond the camera resolution. Hence rejection of electrically functional parts will occur. As lead pitch and feature size continue to be reduced, this problem becomes further aggravated.